Why does concrete crack? Causes, types and prevention

Introduction

Cracks in concrete are a recurring problem in construction. However, not all cracks are the same, nor do they have the same impact on durability or structural safety. To properly address this phenomenon, it is necessary to understand the causes of these failures. This article explores the main reasons why concrete cracks, providing a detailed and technical overview of the types of cracks in concrete, their causes, and preventive and corrective measures to avoid their progression.

Concrete: Key in construction

Concrete, also known as concrete, is an indispensable composite material in construction. It is obtained by mixing Portland cement, coarse aggregates such as gravel, fine aggregates such as sand, water, and, sometimes, special additives that modify its physical and chemical properties. When water is added, a chemical reaction called hydration begins, which transforms the plastic mixture into a rigid and resistant structure, forming a matrix of artificial stone.

This material is especially valued for its high compressive strength, its ability to adapt to complex shapes during its fresh state, and its durability against adverse environmental conditions, such as humidity, freeze-thaw cycles, and chemicals. To increase its capacity to withstand tensile loads, it is reinforced with steel bars, creating reinforced concrete, ideal for withstanding both static and dynamic forces.

Over time, concrete technology has evolved, incorporating admixtures that improve its properties, such as resistance to cracking, flexibility to thermal changes, and waterproofing capacity, making it one of the most versatile and durable materials in modern engineering.

Types of cracks in concrete and their causes

• Structural cracks: These are mainly due to design errors or unforeseen overloads. These originate when stresses exceed the strength of the concrete, which can result from tensile, bending, or shear forces. A common mistake is to underestimate the dynamic loads or not to consider adequately the reinforcement of the reinforcement, causing failures under loads higher than those designed. Axial tensile cracks, for example, can cross the entire section of a structural element, compromising the safety of the structure.
• Cracks due to environmental factors: Changes in temperature and humidity are frequent causes of cracks. Thermal variations generate internal stresses that end up cracking the structure. These stresses occur when the outer layers of concrete expand or contract faster than the inner core, causing surface cracks. In addition, moisture and drying in repeated cycles can weaken the concrete surface, allowing aggressive agents such as salts or acids to enter, accelerating corrosion.
• Shrinkage cracks: Plastic and drying shrinkage is another frequent cause of cracking. These cracks are generated when concrete loses water rapidly, causing a reduction in its volume. This type of failure occurs during the first hours after the concrete is poured when surface water evaporates. Drying shrinkage, on the other hand, appears in the long term and is influenced by the amount of water in the mix and the curing method used.
• Slump cracks: Uneven ground settlement is one of the main causes of structural cracks in concrete. When a structure does not have a solid or uniform base, differential soil movements generate additional stresses that result in cracks at the points of greatest stress. They are very common in areas where the soil is unstable or prone to movement and are easily recognized by their diagonal inclination.

The following video shows the different types of structural cracks that can occur in reinforced concrete slabs and their causes. Including flexural cracks, corrosion cracks, shrinkage cracks, stress cracks, and tension cracks. Causes of these cracks include excessive loads, design or construction deficiencies, and environmental factors. Source: Engineering Infinity

External and internal factors contributing to crack formation

External factors

1. Functional loads: These include all forces acting on a structure throughout its service life. Static loads, such as the structure’s own weight, or dynamic loads, such as those generated by traffic or vibrations, generate continuous stresses on the structure. If not adequately considered in the structural design, the concrete may not withstand the imposed loads, resulting in progressive cracking or failure.
2. Environmental loads: The environment is a determining factor in the integrity of concrete. Structures exposed to drastic changes in temperature, humidity, or phenomena such as wind, snow, and moving water (such as waves) are subject to additional loads. These actions generate stresses in the structure that can lead to cracking if their effect has not been adequately anticipated.

Internal factors

1. Intrinsic changes: Concrete is subject to internal deformations such as plastic shrinkage, thermal shrinkage, and alkali-silica reaction. These deformations occur as a result of cement hydration, moisture loss, or internal chemical reactions. If these factors are not controlled, the resulting stresses can lead to cracking, compromising the strength and durability of the concrete.
2. Induced deformations: Deformations imposed during the service life of the structure, such as prestressing or post-tensioning, are also responsible for cracks in concrete. These techniques are used to improve the structural behavior of concrete, but if not properly executed, they can generate excessive internal stresses that cause cracks. Also, differential settlements, where the supports of the structure are displaced in a non-uniform manner, could generate diagonal cracks in the structures.

Impact of cracks on the durability and safety of structures

Cracks affect the aesthetic appearance of concrete, but their main impact is directly on its structural integrity and durability. Cracks allow the entry of water, salts, chemicals, and other corrosive agents that accelerate the deterioration of the steel reinforcement inside the structure, initiating corrosion processes.

This process weakens the structure and compromises its long-term strength. If corrective measures are not taken in time, cracks in the concrete could evolve into serious structural problems, which increase maintenance costs and shorten the useful life of the structure, putting its operational safety at risk.

Preventive measures

Preventing concrete cracks requires a combination of proper structural design, selection of quality materials, and good construction practices. Among the most effective strategies are:

1. Design and material selection: perform an efficient structural design that considers all possible static and dynamic loads, and environmental factors. In addition, select appropriate reinforcement and ensure that materials meet quality specifications. The choice of an appropriate cement, the use of shrinkage-controlling admixtures, and proper curing are necessary measures to reduce risk.
2. Construction practices: This starts with the use of a quality mix to avoid shrinkage and ensure strength, followed by proper placement and compaction to ensure good adhesion to the reinforcement and avoid voids. Curing must then maintain humidity to prevent water loss and allow complete hydration. It is also required to control the ambient temperature and humidity during the process, and to perform periodic inspections to correct possible problems in time.

By following these recommendations, you can significantly reduce the chances of concrete cracking and ensure the durability of the concrete and safety of the structure.

How can cracks in concrete be repaired?

Prior to any repair, it is necessary to evaluate the cause of the crack to correct the underlying problem and prevent its recurrence. In cases of more severe cracks, non-destructive testing, such as ultrasound or industrial radiography, is recommended to determine internal damage. Techniques vary depending on the type, size, and severity of the crack. Some of the most common repairs include:

• Epoxy resin injection: used for structural cracks. This technique fills and seals the crack, restoring the structural capacity of the concrete. It is effective for cracks that compromise the safety of the structure, such as those occurring in beams or columns.
• Use of flexible sealants: Silicone or polyurethane sealants are effective for repairing small, superficial cracks, such as those caused by shrinkage or wear. These materials are a better choice than rigid materials, as they allow some mobility without breaking and create an impermeable barrier, preventing moisture from penetrating and causing further damage.
• Repair mortars: For deeper or more extensive cracks, repair mortars are used to fill the cracks and restore the affected surface. This technique is common on pavements and surfaces subjected to constant wear.
• Application of additional reinforcement: In some cases, especially when cracks compromise structural safety, it may be necessary to add additional reinforcement, such as rebar or post-tensioned cables, to redistribute stresses and prevent cracks in the concrete from propagating.
• Use of control joints: In some situations, in addition to repair, the installation of control joints is necessary to prevent the occurrence of new cracks caused by concrete shrinkage.

Importance of reinforcement for crack control in structures

Steel bars or prestressing wires distribute loads and absorb stresses, which reduces the risk of cracking. By improving the ability of concrete to withstand tensile and flexural loads, reinforcement decreases the likelihood of structural failure, especially in areas with high loads or exposed to extreme environmental conditions. In addition, reinforcement helps prevent cracks caused by shrinkage and internal stresses.

Conclusions

Although cracks in concrete are a common problem, most can be prevented and repaired with proper action. Understanding why concrete cracks is the first step in ensuring the durability of concrete and the safety of structures. There are several types of concrete cracks, each with specific causes and characteristics that must be properly evaluated and repaired.

The key is a combination of sound structural design, choice of appropriate materials, good construction practices, and proper repair when necessary. Reinforcement techniques are critical to ensure that the structure maintains its integrity over the long term, minimizing the risk of cracks that can be serious and costly.

References

1. Manual de Rehabilitación de Estructuras de Hormigón; Reparación, Refuerzo y Protección. Editors; Pulo Helene, Fernanda Pereira.